Optical Coherence Tomography (OCT) is a technology for performing high-resolution cross sectional imaging that can provide images of tissue structure on the micron scale in situ and in real time (U.S. Pat. No. 5,321,501). In recent years, it has been demonstrated that Fourier domain OCT (FD-OCT), which so far employs either a wavelength swept source and a single detector or a broadband source and an array spectrometer, has significant advantages in both speed and signal-to-noise ratio as compared to time domain OCT (TD-OCT) (Choma, M. A. et al. (2003). “Sensitivity advantage of swept source and Fourier domain optical coherence tomography.” Optics Express 11(18): 2183-2189). In TD-OCT, the optical path length between the sample and reference arms needs to be mechanically scanned. In both swept source OCT (SS-OCT) and spectrometer-based spectral domain OCT (SD-OCT), the optical path length difference between the sample and reference arm is not mechanically scanned. Instead, a full axial scan (also called A-scan) is obtained in parallel for all points along the sample axial line within a short time determined by the wavelength sweep rate of the swept source (in SS-OCT) or the line scan rate of the line scan camera (in SD-OCT). As a result, the speed for each axial scan can be substantially increased as compared to the mechanical scanning speed of TD-OCT and this is especially beneficial for real-time imaging of living biological samples such as the human eye. In addition, SD-OCT and SS-OCT can provide substantially greater signal-to-noise ratio relative to TD-OCT, as explained by Mitsui (1999) “Dynamic Range of Optical Reflectometry with Spectral Interferometry.” Japanese Journal of Applied Physics 38(10): 6133-6137.
SS-OCT can be achieved using either a single lasing wavelength tunable laser or a multiple lasing wavelengths tunable laser. FIG. 1 shows the basic configuration of a SS-OCT system based on a tunable laser with a single lasing wavelength. Light from a tunable single-wavelength laser 102 is split through a beam splitter or fiber coupler 104 into a reference arm 106 and a sample arm 108 of an interferometer and the interference signal is detected with a single high-speed photodetector 110. By sweeping the wavelength of the monochromatic source 102, the interference spectrum from the OCT interferometer is recorded sequentially. The axial reflectance distribution of the sample is obtained by a Fourier transform of the sequentially acquired spectral interference signal.
FIG. 2 shows a system, described in a co-pending US patent application by Zhou and Everett (“Fourier domain optical coherence tomography employing a swept multi-wavelength laser and a multi-channel receiver” filed on Jul. 1, 2005, application Ser. No. 11/174,158) incorporated herein by reference, of a SS-OCT system based on a tunable laser with multiple lasing wavelengths. Light from a tunable multi-wavelength laser 202 is split via a beam splitter, for example, fiber coupler 204, into a reference arm 206 and a sample arm 208 of an interferometer. Light returning from the reference arm and the sample is combined, either with the same splitter as shown in FIG. 2 or another beam combining element as is known in the art of interferometry. The combined, interfered light is sent to a detector, in this case, multi-channel receiver 210. A processor 220 obtains the spectral interferogram data from the multi-channel receiver 210, synchronized with the sweeping of the multi-wavelength laser 202. It combines the data samples from the individual channels to form the full spectral interference fringes and carries out a Fourier transform of the spectral interference fringes to provide the information of the reflectance distribution along the depth within the sample 222.
A practical SS-OCT system requires a high speed swept source with a sweep rate of at least about 20 kHz that is continuously tunable over a broad tuning range (preferably greater than 50 nm). Current commercially available tunable lasers can be divided into electronically tuned lasers and mechanically tuned lasers. Electronically-tuned lasers are either limited in their tuning range (typically 5 nm to 10 nm for a single distributed feedback (DFB) laser), or discretely tunable in order to cover a wider range as in the case of sampled grating distributed feedback reflector (SG-DBR) lasers (see for example, U.S. Pat. No. 4,896,325, U.S. Pat. No. 5,325,392). The discretely tunable lasers described in U.S. Pat. No. 4,896,325 and U.S. Pat. No. 5,325,392 operate using a single gain section, and they tune using a Vernier effect between the two DBR end mirrors, so both DBR end mirrors and a phase-matching device must be simultaneously continuously tuned to produce discrete tuning; these features make this design inconvenient for SS-OCT. Most mechanically tunable lasers are slow. Some use fiber and piezo based Fabry-Perot (FP) filters (see for example Huber, R. et al. (2005) Optics Express 13(9): 3513-3528; and (2006) Optics Express 14(8): 3225-3237) and others use fast rotating polygon mirrors (see for example, US20050035295). For example, patent application US20050035295 and the article by Oh, W. Y. et al. (“Wide tuning range wavelength-swept laser with two semiconductor optical amplifiers.” Photonics Technology Letters, IEEE 17(3): 678-680) disclosed a wavelength tuning source for SS-OCT that employs a continuously rotating optical arrangement for lasing wavelength selection. The current price of a swept source suitable for OCT is very high (see for example, Thorlab Inc. Product Catalog, Vol. 17, (2005) page 469) and in addition, the demonstrated wavelength sweep rate is limited to about 20 kHz.
On the other hand, tunable semiconductor lasers developed for optical fiber communications either are step-tuned to fit the ITU grid (see for example, Amano, T. et al. (2005). “Optical frequency-domain reflectometry with a rapid wavelength-scanning superstructure-grating distributed Bragg reflector laser.” Applied Optics 44(5): 808-816) or, if continuously tunable, are very slow (see for example, U.S. Pat. No. 6,847,661) and they do not meet the requirement for an SS-OCT system, such as the high wavelength sweeping rate (more than 20 kHz) and the broad spectral range to be covered (e.g. 25 to 200 nm). Although there are various designs of semiconductor based tunable lasers (see for example, Muller, M. et al. (2003) “1.3-μm Continuously Tunable Distributed Feedback Laser With Constant Power Output Based on GalnNAs—GaAs”, Photonics Technology Letters, IEEE 15(7) 897-899; Buss J. et al. (2005) “Tunable Laser Diodes and Related Optical Sources” Second Edition, John Wiley & Sons, Inc., Hoboken, N.J., and others as cited in this application, which are all incorporated in their entirety herein by reference), these lasers are not designed specifically for SS-OCT applications. In particular, there are attempts to cascade a few distributed feedback (DFB) semiconductor lasers along a single channel waveguide to achieve complex coupled DFB lasers (see for example, U.S. Pat. No. 5,936,994; U.S. Pat. No. 6,104,739; U.S. Pat. No. 6,201,824; Hong, J. et al. (1998) “Enhanced Wavelength Tuning Range in Two-Section Complex-Coupled DFB Lasers by Alternating Gain and Loss Coupling”, Journal of Lightwave Technology, 16(7): 1323). When the individual sections of these lasers are built on semiconductor structures having uniform energy band gap, there is a significant overlap of the optical gain curve associated with each DFB grating and the resulting lasers have limited tuning range.
In light of the above, there is hence a need in the art for a low cost continuously tunable laser that meets the requirement of a real time SS-OCT system.